Semiconductor device fabrication involves a number of processes where temperature uniformity and control are critical. These processes include rapid thermal processing (RTP), chemical vapor deposition (CVD), physical vapor deposition (PVD), and plasma etch. The ability to measure and map the silicone wafer temperature during processing is an enabling technology for most modem processes, especially RTP.
Modern techniques for remote monitoring of semiconductor wafer temperature are generally ineffective. While thermocouples provide adequate precision, they require physical contact with the wafer for accurate measurement and hence, they disturb the temperature field and uniformity, and further provide a source of contamination. Optical pyrometry is a state-of-the-art technique for remote temperature analysis. However, pyrometers are inaccurate primarily because the temperature measurement is a strong function of emissivity which varies greatly with wafer coating, film growths, and depositions. Pyrometers are particularly ineffective at low temperatures. Furthermore, at high temperatures, heat lamps are commonly used during RTP. They generate bright light to heat the wafers with infrared radiation, which can cause false pyrometric readings.
Laser ultrasonics has recently been introduced as a means for remote temperature measurement of thin materials as described in U.S. Pat. No. 5,604,592, incorporated herein by reference. In this technique, a stimulus beam is incident on a portion of the silicon wafer.
The stimulus beam generates an ultrasonic, or elastic, stress wave which propagates along the body of the wafer. The elastic wave is remotely sensed by a sense beam at a sense location positioned at a known distance from the source location. The velocity of propagation between the stimulus location and the sense location is temperature-dependent. In this manner, the temperature of the wafer is determined as a function of propagation time, referred to in the art as "time of flight" (TOF) of the elastic wave. At present, the accuracy of laser ultrasound is limited to .+-.4.degree. C. and therefore this technique is ineffective for modem RTP applications where an accuracy of .+-.3.degree. C. is necessary and .+-.1.degree. C. is desired. Furthermore, laser ultrasound is extremely sensitive to wafer thickness.